Fresh water is required for a wide range of application including industrial usage and daily-life usage, and the demand for fresh water is increasing. As a supply source of fresh water, an extensive seawater desalination system has been developed to utilize abundant seawater to produce freshwater. This type of seawater desalination system requires pretreatment for removing suspended solids and microbes, before separating salt from seawater with a filtering device such as a reverse osmosis membrane, a NF membrane, and an electrodialytic membrane.
Such pretreatment is typically performed by adding chemicals, including chlorine (Cl) as a sterilizer, ferric chloride (FeCl3) as flocculant, and sulfuric acid (H2SO4) for pH control, for instance. However, use of chemicals in pretreatment leads to an increase in desalination cost including running cost. Accordingly, Patent Document 1, for instance, discloses a technique for avoiding use of chemicals and reducing running cost, by using a sand filter with a biofilm grown and maintained on a sand surface to remove particles contained in seawater as well as silt density index (SDI) components (e.g. particles and colloid) which contaminate a film surface and BOD components which cause biofouling.
FIG. 11 is a schematic configuration diagram of a seawater pretreatment device 10 disclosed in Patent Document 1. The pretreatment device 10 is a device for performing pretreatment before separating fresh water with a device 14 for removing or concentrating salt in seawater by membrane filtration, and includes a plurality of sand filters 16. Each sand filter 16 removes particles from seawater through sand 18 serving as a particle filtering material, and also removes SDI components and BOD components by using a biofilm 20 grown and maintained on the surface of the sand 18. Especially in Patent Document 1, a plurality of the sand filters 16 is prepared (primary sand filter 16a, secondary sand filter 16b), and connected in series to a flow channel of seawater.
Each sand filter 16 of the pretreatment device 10 is equipped with a back-cleaning pump 15 which performs reverse-current cleaning to clean a filtering material in case a differential pressure of the filter increases. Seawater having flowed through the pretreatment device 10 as described above is separated from concentrate seawater and desalinated by the device 14 for removing or concentrating salt in seawater by membrane filtration.